Ignition of an air-fuel mixture gas in an internal combustion engine is normally performed by spark discharge using an ignition plug. However, there have been proposals for a microwave ignition system that uses electromagnetic waves with a frequency of several gigahertz (GHz); i.e., microwaves, for improving combustion efficiency and other purposes.
In order to reliably ignite the air-fuel mixture gas only using microwaves, an extremely strong electrical field is required; therefore, there is a need to put a cavity in a resonant state. As an attempt to resolve this problem, it was proposed in Patent References 1 and 2 for a stub structure for regulating the resonance frequency to be provided in a combustion chamber. It was proposed in Patent Reference 3 for a resonance structure to be provided in the plug in order to obtain a strong electrical field.
Means for combining spark discharge and microwave energy is proposed in patent reference 4. According to a technique described in patent reference 4, an electrode for the spark discharge and an electrode for receiving the microwave energy are each provided separately in a cavity, and are configured so that a strong electrical field is formed in a region of the spark discharge electrode that is physically distant from the electrode for radiating microwaves.    Patent reference 1: JP-A 2000-230426    Patent reference 2: JP-A 2001-73920    Patent reference 3: JP-A 2004-87498    Patent reference 4: JP-A 2000-221019